                (a) Field of the Invention        
This invention relates to the field of vehicle seat systems equipped with an active headrest mechanism for moving a headrest to front of a seat when receiving a load from the seat occupant in the event of a rear-end collision with another vehicle, and a lumbar support mounted on a seatback frame in a seatback for position adjustment in the front and back direction of the seat.
(b) Description of the Related Art
Active headrest mechanisms have been known that reduce whiplash injuries of seat occupants in rear-end collisions with another vehicle such as a car. The whiplash injury reduction is implemented by moving a headrest at the top of a seatback to front of the seat (i.e., moving toward the head of the seat occupant) when a load from front of the seat acts on a load-receiving part placed in the seatback (see, for example, Japanese Patent Application Publication No. 2000-325179). Such an active headrest mechanism is required, in rear-end collisions, to early move the headrest to front of the seat (toward the seat occupant) and obtain a large amount of the movement. To satisfy this requirement, it is preferable to place the load-receiving part substantially at the same height as the seat occupant's lumbar as disclosed in Japanese Patent Application Publication No. 2000-325179.
Some of vehicle seat systems have a lumbar support provided in the seatback substantially at the same height as the seat occupant's lumbar to support his lumbar, and the lumbar support is adjustable in position in the front and back direction of the seat. Further, there are also known vehicle seat systems into which an active headrest mechanism as described above is assembled together with such a lumbar support (see, for example, Japanese Patent Application Publication No. 2002-345595). In such a vehicle seat system, a lumbar support plate for supporting the seat occupant's lumbar is mounted at its bottom end to a seatback frame, a crank-shaped rod is engaged with the vertically middle part of the lumbar support plate, and the lumbar support plate can be moved in its vertical middle (i.e., at the same height as the seat occupant's lumbar) in the front and back direction of the seat relative to the seatback frame by the crank-shaped rod. Further, a headrest support member spans the top ends of both side frames of the seatback frame for pivotal movement about an axis extending in the lateral direction of the seat. Furthermore, a load-receiving member is attached at one end to the headrest support member so that the other end adjoins the top end portion of the back face of the lumbar support plate (the surface thereof toward the back of the seat) to form a load-receiving part. Thus, when a large load is applied from the seat occupant to the top end portion of the lumbar support plate in a rear-end collision, the load is transported through the lumbar support plate to the load-receiving part, so that the headrest support member pivotally moves to move the headrest to front of the seat.
In the known vehicle seat systems with an active headrest mechanism and a lumbar support, however, it is difficult to place the load-receiving part of the active headrest mechanism substantially at the same height as the seat occupant's lumbar. Specifically, where the load-receiving part is placed on the vertical middle portion of the back face of the lumbar support plate, the vertical middle point of the lumbar support plate varies in the front and back direction of the seat according to the position adjustment of the lumbar support plate in the front and back direction of the seat. As a result, a load in a rear-end collision may not act on the load-receiving part depending on the position of the lumbar support plate or the headrest may be unwantedly moved by the position adjustment of the lumbar support plate. In order to solve these problems, it is necessary to vertically elongate the lumbar support plate and place the load-receiving part on the top end portion of the back face of the lumbar support plate, i.e., away from the seat occupant's lumbar, as disclosed in the above-mentioned Japanese Patent Application Publication No. 2002-345595. With this arrangement, however, load input to the load-receiving part in a rear-end collision is delayed and the amount of movement of the headrest is small.